1. Field of the Invention
The present invention relates to a power converting apparatus which is used for an elevator control device and an electric vehicle control device, and more particularly, the present invention pertains to a power converting apparatus comprising self turn-off type semiconductor devices.
2. Description of the Related Art
FIG. 7 is a circuit diagram of a voltage type inverter such as, for example, an inverter for use with an elevator control device.
The input terminal of a rectifier 2 is connected to a three phase alternating current (AC) power source 1, while the output terminal is connected to an inverter 4 through a condenser 3. Inverter 4 comprises a plurality of self turn-off type semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z, such as bipolar transistors (Tr), giant transistors (GTR), insulated gate bipolar transistors (IGBT), and gate turn off thyristors (GTO). Semiconductor device 4u is connected in series with semiconductor device 4x. Semiconductor device 4v is connected in series with semiconductor device 4y. Semiconductor device 4w is connected in series with semiconductor device 4z. A U-phase terminal of an induction motor 5 is connected between semiconductor device 4u and semiconductor device 4x, a V-phase terminal is connected between semiconductor device 4v and semiconductor device 4y, and a W-phase terminal is connected between semiconductor device 4w and semiconductor device 4z. A current detector 6 detects the current which flows through condenser 3.
Three phase AC power source 1 provides three phase AC voltage for rectifier 2. Rectifier 2 converts the three phase AC voltage to direct current (DC) voltage. Condenser 3 smoothes the DC voltage. Semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z are controlled "on" or "off" by an on/off signal provided from control device 100. For example, semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z are switched according to the following order 4u, 4w, 4y.fwdarw.4u, 4z, 4y.fwdarw.4u, 4z, 4v.fwdarw.4x, 4z, 4v.fwdarw.4x, 4w, 4v.fwdarw.4x, 4w, 4y.
Semiconductor devices 4u, 4x are controlled so as not to be switched on simultaneously. Semiconductor devices 4v, 4y are controlled so as not to be switched on simultaneously. Semiconductor devices 4w, 4z are controlled so as not to be switched on simultaneously.
FIG. 8 is a circuit diagram of control device 100. For example, control of semiconductor device 4u which is controlled "on" or "off" will be explained.
Control device 100 has a first side control device 7, an isolation device 12, and a second side control device 13.
First side control device 7 has a source of electricity P15, a control signal outputting circuit 8, a transistor driver 9, and limiting resistors 10, 11. Control signal outputting circuit 8 outputs a control signal to switch on semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z. The control signal is provided from an external control circuits (not shown) such as a speed control circuit to control the rotation speed of induction motor 5, a current control circuit to control the current supplied to induction motor 5, and a pulse width modulation control circuit. Transistor driver 9 switches on when control signal outputting circuit 8 outputs the control signal.
Isolation device 12 isolates first side control device 7 from second side control device 13. For example, isolation device 12 is a photo-coupler comprising a light-emitting device 12a and a photo detector 12b.
Second side control device 13 has a current amplifying circuit 14, a limiting resistor 15, and condensers 16a, 16b.
Current amplifying circuit 14 has transistors 14a, 14b. Transistor 14a is connected in series to transistor 14b. Limiting resistor 15 is connected between the connection of transistors 14a, 14b and a gate terminal of semiconductor device 4u. Condenser 16a is charged positively. Condenser 16b is charged negatively. Condensers 16a, 16b supply positive voltage or negative voltage to semiconductor device 4u.
When control signal outputting circuit 8 outputs the control signal provided from the external control circuits (not shown), transistor driver 9 switches on. Then current flows in the following circuit path of source of electricity P15--an electric wire 101--light-emitting device 12a--limiting resistor 11--electric wire 101--limiting resistor 10--transistor driver 9. When light-emitting device 12a radiates, photo detector 12b switches on. Then the charge of condenser 16a is discharged through the circuit path of transistor 14a--limiting resistor 15--semiconductor device 4u. As a result, semiconductor device 4u switches on.
However, when the external control circuits does not operate normally and control signal outputting circuit 8 outputs the wrong control signal, transistor driver 9 switches on and the combination of semiconductor devices 4u-4x or 4v-4y or 4w-4z are switched on simultaneously. When source of electricity P15 is picking up noises and light-emitting device 12a radiates in error photo detector 12b switches on, with the result that the combination of semiconductor devices 4u-4x or 4v-4y or 4w-4z are switched on simultaneously. When the combination of semiconductor devices 4u-4x or 4v-4y or 4w-4z are switched on simultaneously, condenser 3 is short-circuited. Therefore the charge of condenser 3 is discharged through the combination of the semiconductor devices 4u-4x or 4v-4y or 4w-4z which are switched on simultaneously momentarily. IN such a case, the semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z may be damaged.
There are the following methods to prevent the semiconductor devices 4u, 4x, 4v, 4y, 4w, 4z from being damaged. Current detector 6 detects the current flowing through condenser 3. A protection device (not shown) receives the current value which is detected by current detector 6. The protection device cuts off the control signal to the combination of the semiconductor devices 4u-4x or 4v-4y or 4w-4z when the current value is more than a predetermined current value. Japanese Patent Disclosure (kokai) No.3-270690 disclose a method to detect the combination of semiconductor devices 4u-4x or 4v-4y or 4w-4z being switched on simultaneously, when the voltage across these semiconductor devices decrease excessively.
However, after the combination of semiconductor devices 4u-4x or 4v-4y or 4w-4z are switched on simultaneously, the control signal provided for these semiconductor device will be cut off. Therefore the current continues to flow through the semiconductor devices until the control signal is cut off. Therefore it is difficult to protect the semiconductor device.
Also when current or voltage is detected with too great a sensitivity, the wrong detection may occur.
A high-speed switching semiconductor device such as an IGBT requires detection of simultaneous device operation at high-speed. However the methods available for reliably protecting such semiconductor devices are not.